Greek National Hail Suppression Program

An important loss prevention activity of EL.G.A. is the implementation of the Greek National Hail Suppression Program (G.N.H.S.P.). The Greek National Hail Suppression Program is a weather modification program implemented in rural areas of Northern and Central Greece. It is an aerial cloud seeding project aimed at mitigating hail damage caused to agricultural production. It is applied every year from March 20 to September 30 over specific areas of Central Macedonia and Thessaly. The program is conducted by specialized meteorological and flight personnel, using meteorological radars to locate hail clouds and specially equipped aircraft to carry out cloud seeding.

The operational procedures of the program include forecasting thunderstorm occurrence and intensity, operating meteorological radars to observe, track, and record storms, and cloud seeding by aircraft equipped with systems for firing and launching Silver Iodide flares within the clouds. Seeding aircraft, guided by meteorologists and radar controllers, release Silver Iodide nuclei in an appropriate area of a thunderstorm and at a time interval associated with the formation and growth of hailstones.

EL.G.A., with increased sensitivity for possible environmental effects of the seeding material (Silver Iodide) for the suppression of hail, assigned relevant research to the Aristotle University of Thessaloniki. The results showed very low concentrations of the substance used to suppress hail, without any hazardous effect on terrestrial and aquatic ecosystems.

The effectiveness of the Program in suppressing hail and consequently reducing damage to agricultural crops has been tested with a set of studies and methods. A statistical evaluation was done with data from the hailpad network and showed a decrease in the average hailstone diameter by 35%, the total number of hailstones by 45%, and the kinetic energy of hailstones by 70%. The financial evaluation, based on data of hail damage financial compensations with a comparison between neighboring areas for the same time period or for the same areas in different time periods, led to similar conclusions.

The National Hail Suppression Program is implemented over two rural areas of northern and central Greece, covering a total surface of approximately 5,200 km² (Figure 1). The selection of the specific areas was based on climatic, geomorphological, and agricultural insurance data.

Protection Area 1 (P1) - Central Macedonia: It includes parts mainly of the prefectures of Imathia and Pella, as well as smaller parts of the prefectures of Pieria, Thessaloniki, and Kilkis, covering an area of approximately 2,740 km² (Figure 2).
Protection Area 3 (P3) – Thessaly: It includes mainly parts of the prefectures of Karditsa, Trikala, and Larissa, as well as a small part of the prefecture of Fthiotida, covering an area of approximately 2,460 km² (Figure 3).

Figure 1. Map of the two Protection Areas of the G.N.H.S.P.

Figure 2. Map of the Protection Area of Central Macedonia (P1).

Figure 3. Map of the Protection Area of Thessaly (P3).

The term "hail" expresses not only the appearance of hailstones on the ground but also the overall hailfall phenomenon. Hailstones develop within Cumulonimbus clouds. The modification of the precipitation production mechanism is done by introducing, in various ways and in specific areas of the cloud, particles that either act themselves as artificial ice nuclei or contribute to the formation of ice nuclei. The process of introducing artificial particles into the cloud is internationally referred to as cloud seeding.

The conceptual seeding model is a set of principles and assumptions based on which the change in the mechanism of growth and evolution of hailstones is expressed. The conceptual model of "beneficial competition" has been adopted in the G.N.H.S.P. According to this model, it is assumed that there is a deficit of natural ice nuclei in the cloud and that the total amount of water in all phases within the cloud is kept constant during the time interval of the seeding.

By dispersing the seeding material in the cloud, a significantly increased number of artificial ice nuclei is produced, which together with the natural ice nuclei "compete" for the available super-cooled water, thus forming a much larger number of hailstones, but also of significantly smaller size. During their fall to the ground, they melt as they pass through higher-temperature atmospheric layers, mostly turning into raindrops and thus causing less damage.

The process of cloud seeding essentially aims at the formation of smaller-sized hailstones, with an increased possibility that they will melt as they fall to the ground. In this way, the reduction of the size and number of hailstones that eventually reach the ground is achieved, and as a result, the destructiveness of hailstones on agricultural crops is reduced.

The Greek National Hail Suppression Program is run by the Meteorological Applications Center (K.E.M.E.), a unit of EL.G.A. based at "MAKEDONIA" Airport in Thessaloniki. The operation of the Program is implemented by experienced meteorologists and flying personnel backed up by appropriate infrastructure.

In terms of equipment, the operation of the G.N.H.S.P. is based on the use of seeding aircraft, weather radars, Wide Area Network (WAN) and Local Area Networks (LAN), an Aircraft Telemetry System, wireless communications between airplanes and the Operations Center, weather forecasting tools, and a hailpad network.

Cloud seeding is carried out with three PA-3IT Cheyenne II turboprop twin-engine aircraft leased by EL.GA. Aircraft are equipped with special racks for placing cloud seeding flares on the belly of the aircraft and the wings. The firing of the flares is done by a special trigger mechanism installed in the cockpit, following the commands of the meteorologist and radar controller and according to the established thunderstorm severity criteria.

Figure 4. PA-3IT Cheyenne II turbo prop seeding aircraft.

Figure 5. Ejectable flares racks on the aircraft’s belly.

Figure 6. Endburning flares rack on the aircraft’s wings.

EL.G.A. employs two (2) 5 cm weather radars, upgraded with a Sigmet digital receiver and equipped with modern IRIS (Interactive Radar Information System) software. One radar is installed at Filiro, Thessaloniki, covering the Central Macedonia Area, and the second is located at Liopraso, Trikala, covering Thessaly. These radars operate 24 hours a day and are used for continuous observation of weather activity to detect cloud formations and support seeding operations.

The data from the two weather radars are centrally transferred to computers connected to a network at the Operations Center of KEME (see Figure 7). Ground-to-air wireless communication for operation conduct occurs in the UHF frequency range.

Figure 7. KEME’s Operations Center.

Seeding operations are supported by weather forecasting and weather radar surveillance, operating 24 hours a day, seven days a week. Whenever thunderstorms with the potential to develop into hailstorms occur in the protection areas, the meteorologist and radar controller decide on aircraft takeoff, which then approaches the area of the developing thunderstorm. If the criteria for seeding a storm cell are met, the aircrew conducts seeding by releasing Silver Iodide nuclei, applying an appropriate seeding technique.

In the Central Macedonia Area, a Hailpad Network has been operational since 1984 for recording hailstorms, which in 2023 included 157 hailpads (see Figure 8). The hailpad is a simple instrument (see Figure 9) on the surface of which hailstones create dents. Hailpads collected with hailstone dents are digitized and analyzed using appropriate software. Consequently, the values of various hail parameters are determined based on calibration equations.

Figure 8. Hailpad network map at Central Macedonia.

Figure 9. Hailpad installed at the Protection Area of Central Macedonia.